drsuapi: Check that the GUID specified in the request is the really the GUID of the NC

drsuapi: Check that the GUID specified in the request is the really the GUID of the NC

Up to now it was possible to specify in the request the GUID of
CN=Computer,DC=domain,DC=tld but DC=domain,DC=tld as the NC's dn and
still avoid the BAD_NC.

This is because the dsdb_find_nc_root for the specified extended dn will
return DC=domain,DC=tld which is the same as the string part of the DN
specified in the request.

Problem was spoted by Andrew Bartlett <abartlett@samba.org>, thanks !

Signed-off-by: Matthieu Patou <mat@matws.net>